About the tagging stuff:
I think more advanced editing (reorganizing, cleaning up, massive renaming..) tasks should require a more advanced (read "complicated" ;) editors like osmeditor or JOSM.

This is also because the massive change of properties of many segments or points is IMHO not what you normally want. The tags should almost anytime not be on the line segments but on ways over the segments.

We have ways. Use them instead of line segments ;-)

Can you give an example, where properties should be applied to lots of objects simultan (except cleaning up old style tags e.g. moving tags from segments to ways)?

But selecting multiple segments could be interesting for selecting them to CREATE a way. My current implementation lets the user click on every line segment to select one after another. Maybe here, a faster method could be implemented.